1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to a method, a medical device, a computer program product and a computer readable medium for operating an implantable medical device to obtain substantially synchronized closure of the mitral and tricuspid valves based on sensed heart sounds.
2. Description of the Prior Art
Auscultation is an important diagnostic method for obtaining information of the heart sounds, which is well established as diagnostic information of the cardiac function. The sounds are often described as S1-S4. During the working cycle of the heart mechanical vibrations are produced in the heart muscle and the major blood vessels. Acceleration and retardation of tissue cause the vibrations when kinetic energy is transformed to sound energy, e.g. at valve closing. Vibrations can also arise from turbulent blood flow, e.g. at stenosis and regurgitation. These vibrations may be listened to using a stethoscope or registered electronically using phonocardiography, i.e. graphical registration of the heart sounds by means of a heart microphone placed on the skin of the patient's thorax. Auscultation using a stethoscope is, to a large extent, built on practical experience and long practice since the technique is based on the doctor's interpretations of the hearing impressions of heart sounds. When applying phonocardiography, as mentioned above, a heart microphone is placed on the skin of the patient's thorax. In other words, it may be cumbersome and time-consuming to obtain knowledge of the heart sounds and the mechanical energy during the heart cycle using these manual or partly manual methods and, in addition, the obtained knowledge of the heart sounds may be inexact due to the fact that the knowledge is, at least to some extent, subjective.
The first tone S1 coincides with closure of the Mitral and Tricuspid valves at the beginning of systole. Under certain circumstances, the first tone S1 can be split into two components. An abnormally loud S1 may be found in conditions associated with increased cardiac output (e.g. fever, exercise, hyperthyroidism, and anemia), tachycardia and left ventricular hypertrophy. A loud S1 is also characteristically heard with mitral stenosis and when the P-R interval of the EKG is short. An abnormally soft S1 may be heard with mitral regurgitation, heart failure and first degree A-V block (prolonged P-R interval). A broad or split S1 is frequently heard along the left lower sternal border. It is a rather normal finding, but a prominent widely split S1 may be associated with right bundle branch block (RBBB). Beat-to-beat variation in the loudness of S1 may occur in atrial fibrillation and third degree A-V block.
The second heart sound S2 coincides with closure of the aortic and pulmonary valves at the end of systole. S2 is normally split into two components (aortic and pulmonary valves at the end of systole) during inspiration. Splitting of S2 in expiration is abnormal. An abnormally loud S2 is commonly associated with systemic and pulmonary hypertension. A soft S2 may be heard in the later stages of aortic or pulmonary stenosis. Reversed S2 splitting (S2 split in expiration—single sound in inspiration) may be heard in some cases of aortic stenosis but is also common in left bundle branch block (LBBB). Wide (persistent) S2 splitting (S2 split during both inspiration and expiration) is associated with bundle branch block, for example, right bundle branch block, pulmonary stenosis, pulmonary hypertension, or atrial septal defect.
The third heart sound S3 coincides with rapid ventricular filling in early diastole. The third heart sound S3 may be found normally in children and adolescents. It is considered abnormal over the age of 40 and is associated with conditions in which the ventricular contractile function is depressed (e.g. CHF and cardiomyopathy). It also occurs in those conditions associated with volume overloading and dilation of the ventricles during diastole (e.g. mitral/tricuspid regurgitation or ventricular septal defect). That S3 may be heard in the absence of heart disease in conditions associated with increased cardiac output (e.g. fever, anemia, and hyperthyroidism).
The fourth heart sound S4 coincides with atrial contraction in late diastole. S4 is associated with conditions where the ventricles have lost their compliance and have become “stiff”. S4 may be heard during acute myocardial infarction. It is commonly heard in conditions associated with hypertrophy of the ventricles (e.g. systemic or pulmonary hypertension, aortic or pulmonary stenosis, and some cases of cardiomyopathy). The fourth heart sound S4 may also be heard in patients suffering from CHF.
Thus, the systolic and diastolic heart functions are reflected in the heart sound. The heart sounds and their relation carry information of the workload and status of the heart. For example, as discussed above, patients with a wide QRS complex due to e.g. right bundle branch block (RBBB), left bundle branch block (LBBB), or A-V block are associated with a widened or split first heart sound S1. Knowledge of the heart sounds and the mechanical energy during the heart cycle can be used both for diagnosis/monitoring and controlling therapy of such patients. In case of right bundle branch block (RBBB) or A-V block, the patient is normally stimulated with RV or VV pacing intervals selected on basis of echocardiographic studies performed to determine the settings resulting in the best hemodynamic response. After evaluation of hemodynamic effect of varying combinations of pacing intervals, a physician must manually select and program the desired parameters and assume that the optimal setting of the device remain unchanged until a subsequent reoptimization visit. Therefore, automated systems for selecting pacing intervals would be useful in presence of cardiac conditions such as right bundle branch block (RBBB), left bundle branch block (LBBB) or A-V block.
The known technique presents a number of automated systems for controlling/optimizing stimulation therapy as, for example, U.S. Pat. No. 6,792,308, which discloses an implantable medical device, such as a cardiac pacemaker, adapted to sense first and second heart sounds and to optimize the A-V interval using the detected first and second heart sounds. In WO 2004/078257, a method and apparatus for monitoring left ventricular cardiac contractility and for optimizing a cardiac therapy based on left ventricular lateral wall acceleration are disclosed.
However, the prior art does not disclose a method for controlling the stimulation therapy in patients suffering from a wide QRS complex caused by, for example, right bundle branch block (RBBB), left bundle branch block (LBBB) or A-V block on an automated basis using detected or monitored heart sounds.